The present invention relates to the detection and analysis of motion. It is particularly, but not exclusively, concerned with detection of motion measured by interferometer or encoder systems.
It is known to detect movement using interferometry techniques that provide a sensitive way of detecting even small displacements. EP-A-0753804 and GB-A-2296766 disclose examples of such interferometer measuring systems. Similarly, encoders based on the overlap of a scale with an array of sensors can provide accurate measurement. The scale comprises a series of marks. As the marks move relative to the sensors, the relative displacement between the scale and the sensors can be measured on the basis of the output of the respective sensors.
In such systems, the simplest case is where there are two sensors which differ by 90° in phase, for example one producing a sine output whilst the other produces a cosine output. If these are presented on mutually orthogonal axes, the possible outputs can be considered as being within a detection circle centred on the origin of the co-ordinate axes—a Lissajous figure.
At its simplest, that co-ordinate system may represent displacement.
A given point within the circle may be represented by two polar co-ordinates: angle and radius. The radius is a measure of the magnitude of the signal strength, and the angle is a measure of displacement. As the angular ordinate increases or decreases the displacement measured by the system increases or decreases. The displacement resolution of the system is limited by the precision to which the angular ordinate can be measured.
Suppose now the signals are sensed at periodic time intervals. The value obtained at any sensing point will be somewhere within the detection circle. Consider the circle to be divided into 90° segments. In conventional systems the current position (angular segment) is compared with the previous position. There are four possible answers. If the segmental difference is zero, then the system assumes that the system has remained “stationary”. If the difference is +1 then the system assumes that the displacement has positively increased, and if the difference is −1 then the system assumes that the displacement has negatively increased. Finally if the difference is either +2 or −2 then the system flags an over speed error since the system cannot decide whether the system has moved in the positive or negative direction. There are two possible reasons why the system flags a movement by two segments: either the system is moving at sufficient speed to generate a two segment displacement, in which case the system flags a genuine over speed error; or the signals being processed are noisy, in which case the system flags an over speed error inadvertently. These difficulties can be reduced by processing in smaller segments, but again there is always one segment which does not carry sufficient information about how it is reached.